U.S. patent application Ser. No. 231,631 and U.S. Pat. No. 4,408,186 disclose means for providing power line communication over ground and neutral conductors of plural power line branch circuits. This application and patent are incorporated herein for purposes of reference.
In distributed control systems, such as used for energy management and load shedding functions, communication between various elements of the system is usually required. Digital control devices, such as microprocessors, typically communicate by a serial data stream over a pair of conductors where the bit symbols are defined by high or low voltage states on this pair of conductors. This is known as "baseband" communication of the digital data. Baseband communication requires a dedicated pair of conductors, known as a "baseband bus", connected between the various digital elements of a control system for this communication. It has been often found desirable to use the baseband data signals to modulate higher frequency carrier signals for communication, known as "broadband" communication. The carrier signals may be impressed upon the conductors of the power system for distribution within the desired control area. This technique is commonly known as "power line carrier" communication. Other broadband communication carrier frequencies may be used with direct radiation as in radio communication or in infrared or optical communication, and lower frequencies may be used in acoustic or ultrasonic communication. The device which converts baseband data signals to broadband (modulated carrier) data signals, and for conversion from broadband to baseband data signals, is known as a "transceiver". In a typical distributed digital control system, the individual digital control elements, such as microprocessors, exchange baseband data signals with individual transceivers which transfer the baseband data via broadband on a commonly connected single broadband data bus. Thus there are a plurality of baseband buses interconnected by a broadband bus.
In a distributed control system, it is sometimes desirable to interconnect several microprocessors on a single baseband bus and couple that baseband bus through a transceiver to the broadband bus. This reduces the number of transceivers required in a system, hence reducing system cost, but further requires that the transceiver have a novel automatic transmit/receive function.
In such a system, it has been found highly desirable that the various baseband buses appear to be directly interconnected in terms of their exchange of baseband signals, preserving the supreme and inferior characteristics of the two states of the baseband. A transceiver which will provide this apparent interconnection of the baseband buses is known as a "transparent" transceiver because: (1) a supreme state signal impressed upon the baseband input to one transceiver will create a like signal in the baseband output of all other transce-ivers commonly interconnected on the broadband bus, and (2) an attempt to impress an inferior state signal upon the baseband input to one transceiver will only create that inferior state signal in the baseband of all other transceivers commonly interconnected on the broadband bus if no other baseband is in the supreme state.
In order to provide orderly communication, as between the various devices interconnected on a common baseband bus, various techniques or protocols have been developed. In the "polling" protocol, a single master device sends messages to selected individual distributed devices which enables the selected individual device to respond on the bus. In "token passing" systems, a transmission-enabling message is sent from one device to another in an orderly sequence thereby enabling each device to transmit on the bus in turn. In the "multiple access" systems, each device is permitted spontaneous transmission provided that a "listen-before-talk" procedure is followed to prevent transmission when the bus is active with a transmission from another device. Multiple access is preferred in a distributed control system because it offers a faster system response and does not require a master system controller. However, even though each device follows the "listen-before-talk" protocol, it is possible that two devices may start message transmission simultaneously. This condition is known as "contention" for use of the bus. Contention usually results in the destruction of data in the contending messages, known as "collision". It is, therefore, highly desirable in a multiple access system that contention be resolved, eliminating all transmitters but one, prior to collision, in order to avoid the destruction of the data. Contention resolution with collision avoidance results in faster system response and greater system reliability.
In broadband communication, the carrier signal may be modulated by the data signal in amplitude, frequency or phase. These modulation techniques are respectively known as amplitude shift keying (ASK) (also called on-off-keying when the amplitude modulation is 100%), frequency shift keying (FSK), and phase shift keying (PSK). Many different transceiver concepts for the various modulation techniques are currently available, however none show the characteristics of transparency as described above.
The purpose of this invention is to provide a transceiver which provides automatic transmit/receive control and a transceiver which provides the transparent exchange of digital data between a baseband data bus and a broadband data bus, thereby permitting contention resolution with collision avoidance.